1659.5.5 UB and MM Differentiate from a Caudal T+ Population
at Different Times
A lineage tracing experiment utilizing Osr1-GFPCreER mice revealed that Osr1-
positive cells at E6.5 give rise to a small part of the ureteric epithelium. Additionally,
Osr1-positive cells from E7.5 to E8.5 contribute to the metanephros, and those pres-
ent after E9.5 give rise to metanephros other than the ureteric epithelium (Mugford
et al. 2008b). However, because Osr1 is continuously expressed from the early IM
(E8.5) to the late IM (E9.5), it remains unclear whether the bipotent progenitor cells
which give rise to both the MM and UB exist.
We recently employed T-GFPCreER mice to show that T-positive cells at E6.5
and E7.5 give rise to both the MM and UB. Meanwhile, at E8.5 these cells contrib-
uted to the MM but not the UB. The T-positive cells at E9.5 contributed exclusively
to the tail (Taguchi et al. 2014 ). Because expression of T is a marker of immature
caudal tissue, our results indicate that the timing of differentiation from the T+ popu-
lation is different between the UB and the MM. Therefore, we concluded that the
UB differentiates from the T+ population between E7.5 and E8.5 to give rise to the
anterior IM (the early-formed IM), and the MM differentiates from E8.5 to E9.5 T+
cells and develops via the PIM (the late-formed IM) (Figs. 9.5 and 9.6).
The Wolffian duct (WD), also known as the pronephric or mesonephric duct, is
the precursor of the UB, and its morphogenic process has been well studied in
chicken and amphibians (Drawbridge et al. 2003 ). Previous reports have shown that
the WD originates from anterior IM at the axial level of somites 8–10, and commit-
ted WD progenitors migrate toward the cloaca in an anterior-to-posterior direction.
The equivalent WD progenitors in the mouse are proposed to be in the IM at the
level of somites 6–8. These progenitors are identified by the well-conserved mark-
ers, Osr1, Lim1, Pax2 (Pax8), and Ret.
Taken together, these findings indicate that at E8.5 precursors of the UB are
located in the Osr1+/T− population and are already segregated from precursors of the
MM, which are localized in the T+ posterior nascent mesoderm (Figs. 9.5 and 9.6).
Additionally, we successfully induced metanephric nephron progenitors from a
sorted E8.5 caudal T+ population. To this end, we employed a high concentration of
Wnt agonist in combination with Bmp4 to mimic the conditions within the posterior
tail bud and showed maintenance of the T+/Cdx2+ state and induction of posterior
Hox gene expression with time. Treatment with activin, Bmp4, an intermediate con-
centration of Wnt agonist, and retinoic acid synergistically induced a PIM-like pop-
ulation that expressed Osr1, Wt1, and Hox11. Importantly, the depletion of each
single factor or administration of inhibitors hampered nephron progenitor induc-
tion, suggesting the requirement of all signals for this step. Finally, the maturation
factors (Fgf9 + low Wnt) identified in the E9.5 PIM culture successfully induced the
Six2+/Pax2+/Gdnf+ metanephric nephron progenitors in vitro (Fig. 9.7).
9 Early Kidney Specification and Its Recapitulation by Pluripotent Stem Cells